Axial eye length measurements based on Michaelson interferometer arrangements are known. Generally, such apparatus include a beam splitter that 1) projects partially coherent light into a test arm onto a subject's eye, and 2) projects partially coherent light into a reference arm onto a moveable mirror. By moving the mirror a known amount and measuring an output including combined light reflected from the eye and from the mirror, portions of the output that are influenced by interference of light reflected from the eye and light from the mirror can be identified, and various eye lengths can be measured.
Numerous apparatus have been designed to implement such eye length measurement techniques. FIG. 1 illustrates apparatus 100 in which light from a source 110 is projected onto a beam splitter 120 which projects light onto an eye E in a test arm 130 and onto a scanning mirror 140 in a reference arm 150. An output signal from a detector 160 is generated by the combined reflections from the eye and mirror. The amplitude of the signal will increase and decrease due to interference (i.e., interference spikes will arise) when the length of the reference arm is within a distance equal to the coherence length of the light (as determined by the source) of a length in the test arm. A length of the test arm is determined by a reflective surface in the eye (e.g., a surface of the cornea or a surface of the retina). The amount which the mirror is moved between a location to achieve an interference output for a first surface (e.g., a surface of the cornea) and a location to achieve an interference output for a second surface (e.g., a surface of the retina) indicates the distance between the first surface and the second surface. An eye's overall axial length can be measured in this manner.
In apparatus as illustrated in FIG. 1, it is desirable that mirror 140 be moved in a highly linear manner over a substantial length (e.g., 15-35 mm) to obtain accurate eye length measurements. Also, the time required to move the mirror can be substantial, particularly if multiple measurements of a given patient's eye length are to be made and averaged together. As a result, eye movement during measurement can be a source of error.
FIG. 2 illustrates another eye length measurement apparatus 200 in which light from a source 210 is projected onto a beam splitter 220 which projects light onto an eye E in a test arm 230 and into a reference arm 250. However, in contrast to the apparatus of FIG. 1, the reference arm includes two movable mirrors 240a and 240b. A rotating diaphragm 255 in the reference arm is designed to alternately pass light to the first mirror 240a and second mirror 240b. An output signal from a detector 260 is proportional to a light input signal generated by the reflections from the eye and one of the mirrors. The mirrors are provided in a housing 270 that is scanned back and forth (as indicated by an arrow A) toward and away from beam splitter 220. The amplitude of the signal will increase and decrease due to interference when the length of either or both portions of the reference arm is equal to a length in the test arm that corresponds to a reflective surface of the eye.